It's a really tough decision. I had similar issues with my old blind kitty - and same situation. I had an out-of-town wedding and wasn't sure what to do. My sister actually stayed at my house and watched her while I was gone (with full permission to take her in if it came down to it) and they were fine for the weekend.

I was lucky that Pixar pretty much told me when she was done. One morning I woke up and couldn't find her. She hadn't eaten, wouldn't drink anything, and was hiding (something that attention-crazy cat never did. She was known to bite my nose if I was sleeping and she wanted someone to pet her). So I spent a few hours trying to get her interested in food and when she wouldn't even raise her head I ended up taking her in to the vet.

Three and a half weeks can be a long time for something to change in one direction or another. It's hard to wait until the last second if you're trying to plan around a wedding (especially if you have to fly!) Boarding her could be stressful but the vet's is the best place for her to be, and you never know what might happen in the meantime.